Thursday, April 9, 2015

Dichotomous key (VII.b) - The world of internodes cont'd (surface texture)

Aging bark on speckled alder
Surface texture
Surface texture is highly variable between twigs, though some features are common not between taxa, but rather due to convergent evolution. Most forest dwelling twigs are rather bland in terms of texture on the surface. Sugar maple, American beech, and red oak are all generally glabrous and without epicuticular wax (though red oak does have a light sheen to it). Beyond the boring twigs, a tree can have a papery filament coating its surface, a thin white bloom (boxelder, aspen), or it can be fuzzy to furry (as in speckled alder, staghorn sumac, and slippery elm).

The species that do have specialized features coating their epidermis (bark will protect species later in life, but when young the twig needs a boost from the epidermis to do the protecting), are trying to solve the problem of protection against harsh desiccating winds, insect predators, harsh UV radiation (the wax on succulents like dudleyas are known to be have the highest reflectivity of UV radiation of any biological substance), or material that might build up and clog pores or block photosynthesis (see my post on jewelweed). 

Forsythia twig
Epicuticular wax
Forsythia twigs have a whitish hard sheet that encrusts the bark of twigs and younger branches. As it ages, it cracks and sloughs off in flakes. Most of the twigs show a sharp contrast of between the surface exposed to the sun and the shaded surfaces, with the exposed surface being coated in the protective white layer. 

Boxelder has a similar adaptive strategy, but the white bloom is much more of a waxy coating. On a side note, I've often noticed when burning boxelder twigs that oils seem to surface and the heated twigs become glossy and oily to the touch. 

Fuzzy twig of speckled alder (c.f. top image of maturing speckled alder bark)
fuzzy of staghorn sumac twig, which is absent by 3rd year of growth
Fuzz: A twig can be either glabrous (smooth) or pubescent (fuzzy). As with most things, there is more than one way to skin a cat. I've mentioned the boundary layer before (here), and it comes into play again while talking about a twig's fuzziness. In order to prevent drying out, one way to solve the problem is to prevent drying via convective cooling. If you lick your finger and hold it up to the wind, it feels cold on the side the wind is blowing from. The wind evaporates the liquid, drawing away heat. Now say you licked your finger, put on thin gloves and then held it up to the wind. It would take much longer for the saliva to evaporate - longer still the thicker the glove. The tiny (or large on sumac) hairs prevent wind from wicking away moisture for lenticels. 

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